1. Field of the Invention
The present invention relates to an ultrasonic inspection method and an ultrasonic inspection device. The invention more particularly relates to an ultrasonic inspection method in which an array type ultrasonic probe is used, and an ultrasonic inspection device that includes an array type ultrasonic probe.
2. Description of the Related Art
In recent years, constructional materials and the like are inspected by an ultrasonic inspection method. Such a method having being developed more accurately generates an image inside of an object to be inspected in a short time to inspect the inside of the object, as typified by a phased array method and an aperture synthesis method (refer to, for example, Non-Patent Document 1 (Digital signal processing series, volume 12, “Digital signal processing with measurement sensor” pp 143-186, issued by Shokodo, written by Michimasa Kondo, Yukimasa Ohashi, and Akio Jitsumori)).
The phased array method is based on a principle in which an array type ultrasonic probe having a plurality of piezoelectric elements is used and wavefronts of ultrasonic waves transmitted from the piezoelectric elements interfere with each other to form a synthesized wavefront that propagates. Thus, in the phased array method, timings of transmission of ultrasonic waves from the piezoelectric elements are controlled to delay so that timing of each transmission is shifted, thereby enabling control on incident angles of the ultrasonic waves and also focusing of the ultrasonic waves.
For reception of ultrasonic waves, incident angles of ultrasonic waves that are reflected, and received by the piezoelectric elements are shifted in timing and is summed, thereby enabling control on the incident angles to be formed when the ultrasonic waves are received and also reception of the ultrasonic waves in focused state, as is the case with the transmission.
A linear scanning method and a sector scanning method are generally known as the phased array method. The linear scanning method is such that piezoelectric elements for a one-dimensional array probe linearly scan an object to be inspected. The sector scanning method is such that a direction in which an ultrasonic wave is transmitted or received is changed within a fan-shaped region. If a two-dimensional array probe that has piezoelectric elements arranged in a matrix pattern is used, ultrasonic waves can be three-dimensionally focused on any position and thereby scanning suitable for an object to be inspected can be performed. Each of the two methods allows ultrasonic waves to be scanned at high speed without moving the ultrasonic probe. Also, each of the two methods allows incident angles of ultrasonic waves and the vertical position at a focal point thereof to be controlled without replacing the ultrasonic probe. These methods are techniques which enable inspection to be performed at high speed with high accuracy.
The aperture synthesis method is based on the following principle: when a piezoelectric element transmits an ultrasonic wave in such a manner that the ultrasonic wave spreads in an object to be inspected and the piezoelectric element receives the reflected ultrasonic wave, a defect that is the source from which the received reflected ultrasonic wave derives is present on a circular arc that has a center thereof at the position of the piezoelectric element (that transmitted and received the ultrasonic wave) and has a radius of a distance that the reflected ultrasonic wave propagates. The piezoelectric element transmits an ultrasonic wave and receives the reflected ultrasonic wave while the position of the piezoelectric element is sequentially changed. The ultrasonic waves received by the piezoelectric element at the positions are calculated by an electronic computer so that the ultrasonic waves are represented by circular arcs. Intersections on the circular arcs are concentrated at the position of a defect that is the source from which the reflected ultrasonic waves derive, whereby the position of the defect is specified. The details of the calculation performed by the electronic computer are described in Non-Patent Document 1.
In the methods in which a probe that has a plurality of piezoelectric elements is used, the probe can three-dimensionally receive an ultrasonic wave signal reflected from a defect without a movement of the probe. However, in order to specify the three-dimensional position of the defect on the basis of the reflected ultrasonic wave signal, the three-dimensional position of the defect is estimated on the basis of a two-dimensional image having multiple reflection intensity distributions of waves reflected at locations that are spatially different from each other. Alternatively, the three-dimensional position of the defect is estimated by converting the reflection intensity distributions into three-dimensional data and then three-dimensionally displaying the three-dimensional data.
When the linear scanning method and the sector scanning method based on phased array methods are adopted, multiple two-dimensional reflection intensity images responsive to known scanning pitches can be acquired. Thus, a direction in which a reflected ultrasonic wave appears can be specified by sequentially selectively displaying the two-dimensional reflection intensity images on a screen. However, these methods have limitations when three-dimensional scanning other than the aforementioned scanning is performed.
To cope with this, advancement in computer technology in recent years has made available a technique for performing interpolation on ultrasonic wave signals reflected and received from multiple directions so that image data that indicates points three-dimensionally arranged in a matrix pattern is generated and displaying the image data by volume rendering or surface rendering. In addition, there is a technique for displaying an image as a three-dimensional point group without conversion of the reflected ultrasonic wave signals into data that indicates points arranged in a matrix pattern. Since the techniques are designed such that the data is stored as three-dimensional inspection data, an inspector can confirm the three-dimensional inspection data in any direction after the measurement (refer to, for example, Non-Patent Document 2 (“Development of 3-Dimensional Ultrasonic Testing System “3D Focus-UT””, Japan Society of Maintenology, The fifth scientific lecture meeting, Summary report, 155 (2008), written by Atsushi Baba, So Kitazawa, Naoyuki Kono, Yuji Adachi, Mitsuru Odakura, and Osamu Kikuchi) and Non-Patent Document 3 (Potts, A; McNab, A.; Reilly D.; Toft, M., “Presentation and analysis enhancements of the NDT Workbench a software package for ultrasonic NDT data”, REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION: Volume 19. AIP Conference Proceedings, Volume 509, pp. 741-748 (2000)).
However, it is difficult to determine, only on the basis of such three-dimensional inspection data, whether or not a wave that corresponds to a peak of a reflection intensity distribution is a wave reflected on an end surface or boundary surface of an object to be inspected or is a wave reflected on a defect. Especially, it is difficult even for an experienced inspector to make such a determination for inspection of an object having a complex shape, since reflected ultrasonic wave signals (shape echoes) dependent on the shape of the object appear in great numbers. Thus, software has been developed that allows data (three-dimensional shape data) on the three-dimensional shape of an object (to be inspected) to be displayed together with three-dimensional inspection data. By overlapping and comparing the two types of the data using this software, it is possible to easily determine whether an ultrasonic wave signal is a shape echo or an echo (defect echo) generated from a defect. Data generated by a general-purposed computer aided design (CAD) system is read and used for three-dimensional shape data in many cases (refer to, for example, Non-Patent Documents 2 and 3).